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I am clear that the NEC codes say not to run power cables (e.g. to a TV) inside the wall. And I also know that romex cable is allowed and perfectly safe inside a wall.

Codes aside, what is the actual difference/danger with a power cord inside the wall (vs it being safe/allowed inside a surface mounted conduit/cable concealer)?

I have a situation where I want to hide a power cord behind a soundbar, and there’s no room to install a recessed power plug that would not be visible.

Is the issue just that power cables “may” not be as well insulated? I assume the wire is thick enough to avoid dangerous heat dissipation for the power requirements since the cable came with the product.

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    Most electrical wires(in conduit)/cables are run in walls. Are you talking about extension cables? Surface mounts are usually used when a wall is finished and people don't want to make a mess, depends on local also.
    – crip659
    Aug 27 '21 at 15:26
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    Think a better question is if your insurance company will allow it. Insurance likes to get your money, they like it even more when they can get away from paying a claim. A notarize written okay from insurance would be good to have.
    – crip659
    Aug 27 '21 at 16:10
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    If a power cable is generating significant heat , it is much too small for the load . You can not insulate for heating of an electric power cable. Aug 27 '21 at 16:16
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    Does the speaker actually require 120V? Or is there a way to bring low voltage to it? @Crip659 the fire insurance is hardly cheating you if the illegal+unsafe crud caused the accident. Aug 27 '21 at 17:43
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    @Harper-ReinstateMonica Many speakers have built-in amplifiers, and are powered by mains voltage. If the actual loudspeaker in your home movie theater sub-woofer has peak current transients of 20A or more into low impedance (e.g. 2 ohms) which is nothing unusual, why run separate audio cables that will handle that over a long distance when mains power cables already exist? (Of course if you really want to buy audio signal cables that cost $1,000 per foot, there are people who sell them!)
    – alephzero
    Aug 28 '21 at 11:22
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There's whats safe, and then there's what has been tested, tested, and tested again to the point it has building codes written about it to be safe. Is there any real danger running an extension cord through the wall? No, 99% of the time there's no issue and will never cause a problem. But then again, a lot of building codes deal with that 1% so avoidable accidents don't happen.

The main issue is actually what you brought up - insulation and heat dissipation. Extension cords or even device power cords are not rated to any certain specification, and because of that, you can't "know" that it's safe, and you can't publish guidelines about what type of cords are safe because they are so generic.

You might run a cord through the wall to plug in a TV, but then you move out and someone else decides that would be a good place for a wall mounted heater. So, code says "you can't run extension cords through the wall". Of course that is exaggerated... The other issue is that if there is a short and the cord gets hot - how will it burn and is that likely to ignite nearby materials? Romex has been tested for all of that.

So for a low power device like a TV and soundbar, there is very little risk and you should be fine. They even make passthrough kits that come with two wall plates and a tube to run the cables through.

In wall power kit: https://www.amazon.com/-Wall-Power-Cord-Cable-Kit/dp/B00L9K5D5W

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    I would be a bit wary of the particular kit noted. There's no indication I can see that says that it meets UL (or other NRTL) requirements. If there were to be a fire in the OPs house and this were discovered there, it could give the insurance company grounds to deny a claim for installing uncertified equipment. As noted in the answer, "there's what's safe and there's what been tested and certified safe" emphasis added.
    – FreeMan
    Aug 27 '21 at 15:45
  • Thanks, that makes sense. A follow up question: would it be even safer to modify a romex cable with the soundbar plug and run that “romex with plug cable” through the wall instead of the original power cable?
    – catchdave
    Aug 27 '21 at 15:49
  • @FreeMan Good point, but this same kit is sold in store at Home Depot, so I assumed it was UL listed - almost all of their products are because of the reasons you list. Link was mostly for the pictures.
    – JPhi1618
    Aug 27 '21 at 15:53
  • @catchdave that's a whole new question, please feel free to add it. Asking new different questions in the comments is forbidden. I'm sure you'll get a good answer to it, as well.
    – FreeMan
    Aug 27 '21 at 15:54
  • If it's the same brand kit, then yes, it's probably UL listed. If it's a similar kit, this one may not be. Just a warning for the OP to take into consideration.
    – FreeMan
    Aug 27 '21 at 15:55
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It'll be fine if everything else is fine

The #1 issue with cordage inside walls is thermal - cordage is permitted smaller wires because it's in the open air in a habitable room, where a human will notice wire problems... not packed in the wall where air cannot circulate and where it may be subject to 150F heat from solar gain.

But the sound bar draws, what... maybe 40 watts? Less than an amp on a probably #18 power cord. What could possibly go wrong?

We don't install electrical power systems based on everything going right. We wouldn't need GFCIs, grounding, or even circuit breakers, would we?

But if the speaker has a problem we want it to trip the circuit breaker. Breakers protect two ways: Instant magnetic trip (at 10x breaker rating, 150A or 200A)... or inverse-time thermal trip, the more the overload the shorter the time.

The inverse-time curve is designed to approximate the overheating of appropriately sized, appropriately rated in-wall wiring in the walls. Those wires have thermal inertia and it takes time for them to overheat. The idea is that the breaker will tolerate mild overloads for minutes (toaster + coffee maker) or significant overloads for seconds (large motor startup; power supply inrush) because the wires won't overheat that fast.

We throw a monkey-wrench in that design when we embed in the wall cordage, which has not the insulation thermal rating of other in-wall wire, nor the physical size required - #14 for 15A breaker, and #12 for 20A breaker.

PSA: Beware target fixation on bringing 120V to hard places

I can't tell you how many questions we get where someone is trying to bring serious, real, human-killing 120VAC to the most improbable of locations. And we quiz them further, and it turns out they're trying to power a security camera. That doesn't even need 120V, it needs 5V. But the only way their mind's eye can see to power it, is to bring up 120V and use the usual wall-wart. (outdoors in the rain no less). Do you see the blind-side there? They're not even considering simply extending the low-voltage wall-wart cable, a safe and legal operation which Code treats lightly because of the low voltage.

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    Why does the code think 5V short circuits are unable to start fires?
    – user253751
    Aug 28 '21 at 8:16
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    @user253751 Because the statistical likelihood of things going wrong enough for a 5V DC circuit served by an approved adapter to start a fire is exceedingly low? A significant majority of low voltage applications in that range are also low current (almost always less than 5A, usually less than 2.5A), so the total power involved is pretty low (yes, you can start a fire with less than 1W of power, but it’s not easy unless you’re dealing with highly flammable materials). There’s almost always more risk of the adapter itself starting a fire than the wiring or load on the 5V side of it. Aug 28 '21 at 11:59
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    Re the PSA - POE is a fantastic way to get power to places like cameras. One wire, which you need for data anyway.
    – Criggie
    Aug 28 '21 at 22:00
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    The other problem with 5V is that you can't send much of it very far without either needing an enormous cable or having the cable eat up half of the power... there are other solutions, of course.
    – J...
    Aug 29 '21 at 12:50
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    @catchdave It's fairly unlikely that those "135 W" refer to any kind of real power. Unless said soundbar is either absolutely ginormous or has active cooling (noisy fans!) to get rid of 135 W of heat (that's two incandescent light bulbs, ever noticed how hot those get?), that value is the so-called "music" or "P.M.P.O." power, which has nothing to do with physics and everything to do with marketing (it's a made-up figure).
    – TooTea
    Aug 29 '21 at 18:52
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It's primarily about the insulation - IIRC, mostly about the flame-spread characterization of the insulation - thus, even non-power cables are different for "in-wall" and "not in-wall" applications, because if the characteristics of the outer insulation layer.

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    This is true of Ethernet cable. You get your "regular" stuff, or you pay a premium for "plenum" cable. The wire and insulation inside the jacket are identical, but the plenum cable is rated to be run inside a wall. I'm sure it's 100% the same with power-rated cabling, too.
    – FreeMan
    Aug 27 '21 at 17:11
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    Plenum cables are rated for "plenums" that is, spaces that facilitate air circulation. Plenum rated cables are not required inside walls.
    – Craig
    Aug 27 '21 at 18:10
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    i've never heard of ethernet cable that's ok for in-wall but not plenum, so it's a distinction w/o a difference.
    – dandavis
    Aug 27 '21 at 21:09
  • @dandavis If it doesn't explicitly say it's plenum, it's not plenum rated. You can use Plenum cable anywhere non-plenum can be used, however.
    – Machavity
    Aug 28 '21 at 2:45
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    Riser is the non-Plenum in-wall rating for Cat5e/Cat6 etc. "Usually gray," .vs. "usually blue" (but both can be had in any color if you want to pay extra, so you have to actually read the printing on the cable to be sure.)
    – Ecnerwal
    Aug 28 '21 at 17:10

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